Optical pellicle adhesion system

ABSTRACT

One aspect of the invention comprises a method of assembling an optical pellicle including a pellicle frame and a pellicle membrane in which adhesive is first applied to the pellicle frame, and then solvent in the adhesive is permitted to evaporate. Solvent is then applied to the adhesive on the pellicle frame, and the pellicle membrane is mounted onto the adhesive on the pellicle frame. Normally the step in which solvent is permitted to evaporate involves baking the frame with the adhesive thereon at an elevated temperature. Also, it is normal that as soon as the solvent is applied to the adhesive on the pellicle frame, once the adhesive feels tacky, the pellicle membrane is mounted in place. An additional step to this process involves generation of heat in the pellicle membrane and the adhesive after the membrane is mounted to the adhesive on the pellicle frame, to further bond the membrane to the frame.

BACKGROUND OF THE INVENTION

The present invention relates to optical pellicles, and moreparticularly to a system for mounting the pellicle membrane to thepellicle frame.

In the semiconductor clip industry it is well known that patterntransfer from a photomask to a substrate is accomplished by exposing themask to a light source. During the pattern transfer process, also calledthe photolithographic process, patterns on the photomask are projectedonto the substrate, which has been treated with a photosensitivesubstance. This results in the mask pattern being reproduced onto thesubstrate. Unfortunately, any foreign substance on the surface of themask will also be reproduced on the substrate and therefore willinterfere with proper pattern transfer to the substrate.

To eliminate contamination of the mask surface, a framed, thin membraneknown as a pellicle is mounted on the mask surface, such that thepellicle membrane extends parallel to the mask at a predetermineddistance spaced away from it. Any contamination that would ordinarilyland on the mask surface instead falls onto the pellicle membrane.

Pellicles substantially eliminate the above problem becausecontamination on the pellicle membrane generally will not be projectedonto the substrate. The frame of the pellicle supports the membrane at adistance spaced away from the mask surface so any particles or othercontaminants on the pellicle membrane will be out of focus duringpattern transfer.

The use of pellicles can assure the quality of the resulting chip,thereby improving chip fabrication productivity. Consequently, it is nosurprise that pellicle manufacturing techniques have become increasinglyimportant because high quality pellicles are critical to the success ofthe photolithographic process.

An optical pellicle must be highly transparent and extremely clean inorder to perform its intended function. It is also essential that thepellicle membrane be securely mounted to its frame in order to maintainproper and uniform tension, across the surface of the membrane. Pelliclemembranes are extremely thin, usually less than 5 μm, and therefore areoften quite fragile.

In recent years, photolithography has involved the use of deep UV lightwith wavelengths less than 300 nanometers (nm) in order to permit thefabrication of circuits of increasingly smaller size. These deep UVpellicles are extremely thin, normally less than 2 μm, with thicknessesof 1.2 μm, 0.9 μm, and even 0.6 μm being common. Deep UV pellicles canbe of greater thickness, but it is often difficult to precisely controlthe thickness in such pellicles in order to get the maximum transmissionat a specific wavelength, such as 248 nm. Because these pelliclemembranes are so thin, they are fragile, and can be difficult to mountto the pellicle frame. Moreover, the perfluoro polymers and othermaterials of which they are typically fabricated, usually have very lowsurface energy and are difficult to adhere to the pellicle frame. Theuse of these deep UV pellicles has therefore increased the likelihood ofproblems in adherence between the membrane and the frame.

It is a general object of the present invention to provide a new methodfor manufacturing optical pellicles which overcomes the drawbacks andlimitations of the prior art proposals. More specifically, an object ofthe invention is to develop a manufacturing method which provides animproved system for mounting a free-standing pellicle membrane to thepellicle frame, especially where the membrane is of the deep UV variety.Another object of the present invention is to provide a pellicleadhesion system which is readily suited to be a pail of a highlyautomated, easily repeatable process using materials which are readilyavailable.

SUMMARY OF THE INVENTION

One aspect of the invention comprises a method of assembling an opticalpellicle including a pellicle frame and a pellicle membrane in whichadhesive is first applied to the pellicle frame, and then solvent in theadhesive is permitted to evaporate. Solvent is then applied to theadhesive on the pellicle frame, and the pellicle membrane is mountedonto the adhesive on the pellicle frame. Normally the step in whichsolvent is permitted to evaporate involves baking the frame with theadhesive thereon at an elevated temperature. Also, it is normal that assoon as the solvent is applied to the adhesive on the pellicle frame,once the adhesive feels tacky, the pellicle membrane is mounted inplace.

An additional step to this process involves generation of heat in thepellicle membrane and the adhesive after the membrane is mounted to theadhesive on the pellicle frame, to further bond the membrane to theframe.

Various other features, objects and advantages of the present inventionwill become more fully apparent as this description continues.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of an optical pellicle constructed inaccordance with the present invention;

FIG. 2 is an enlarged fragmentary side elevation sectional view takenalong line 2--2 of FIG. 1, with the thickness of the pellicle membranebeing exaggerated for illustration purposes;

FIG. 3 is a view similar to that of FIG. 2 except that it is not soenlarged; it schematically depicts a laser used for further bonding themembrane to the frame;

FIG. 4 is a view corresponding to that of FIG. 3, except that the laseris shown positioned to cut away the excess portion of the membrane.

FIG. 5 is a view corresponding to that of FIG. 3, showing the use of aprotective bead of hot melt glue, with the laser deleted merely tosimplify the Figure; and

FIG. 6 is a view corresponding to that of FIG. 3, showing the use of aprotective piece of tape, with the laser deleted merely to simplify theFigure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 depict a conventional pellicle 10, including a pellicleframe 12 with a pellicle membrane 14 bonded to it by an adhesive layer19. The pellicle flame 12 is typically fabricated of anodized aluminum,although it could alternatively be formed of stainless steel, plastic orother materials. The pellicle membrane 14 is extremely thin, in therange of about 0.5 μm to 5 μm, and is uniform and highly transparent. Asnoted above, a deep UV pellicle membrane is typically between 0.5 μm to2 μm. Many pellicle membranes are fabricated of nitrocellulose orcellulose acetate, while deep UV pellicle membranes are commonlyfabricated of a fluoropolymer such as Cytop® from Asahi Glass or Teflon®AF fluorocarbon amorphous polymer from DuPont.

Deep UV pellicles typically are utilized at photolithography operationsinvolving wavelengths such as 248 nanometers, and possibly 193nanometers. In the preferred embodiment of the present invention, thedeep UV pellicle is fabricated of poly-perfluoro polymer containing acyclic ether functional group, such as poly-perfluoro cyclo oxyaliphaticpolymer formulation of Cytop® identified as CTX-808-SP2. The numerals"08" identify the solution as an 8% solution. This solution is typicallyspun onto a substrate, the solvent is baked off, and the membrane isremoved from the substrate in a peeling operation.

The membrane may alternatively be formulated of Teflon AF amorphousfluoropolymer from DuPont. Teflon AF fluorocarbon is a family ofamorphous copolymers of perfluoro (2,2-dimethyl-1,3 dioxole)(PDD) andtetrafluoroethylene. Teflon AF fluorocarbon is typically based on2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole with fluorine-containingmonomers. Two polymers which work particularly well are Teflon AF1600and AF1601 fluorocarbon amorphous polymers. This is a solution of TeflonAF amorphous fluoropolymer dissolved in Fluorinert FC-75 from 3M.

In order to bond the pellicle membrane to the pellicle frame, it isnecessary to select an adhesive that will provide sufficient bonding butwill not destroy or damage the integrity of the membrane, which isusually less than 2 μm (2×10⁻⁶ meter) thick. The bonding of deep UVpellicle membranes to their frames has been a particularly troublesomeproblem ever since fluoropolymer deep UV pellicles were first introducedseveral years ago. With conventional nitrocellulose or cellulose acetatemembranes, adhesives such as light sensitive Norland NOA 61 has beenentirely satisfactory, but this type of adhesive is ineffective with thefluoropolymer materials used in deep UV pellicles.

It has been found that a suitable adhesive for the materials used indeep UV pellicle membranes is the pellicle material itself except with alower Tg. Thus, for example, for a membrane fabricated of poly-perfluoropolymer containing a cyclic ether functional group, a suitable adhesivewould be a fluorinated polymer containing a cyclic ether functionalgroup. Or, a suitable adhesive for the poly-perfluoro cyclo-oxyaliphaticpolymer membrane described above, is a poly-perfluoro cyclo oxyaliphaticformulation designated by Asahi as CTL-816-AP, which is an analog of themembrane solution CTX-808-SP2. The solid CTL material does, however,have a lower Tg than the CTX material. Alternatively, fluorinated cyclicether polymer can be utilized as an adhesive. The numeral "16"identifies this as a 16% solution, which is typically in a solventdesignated by Asahi as CT-solvent 180. The adhesive is typically appliedto the pellicle fraime as it is provided from Asahi.

The adhesive is applied to the pellicle frame, typically in an automateddispensing operation, and the solvent in the adhesive is allowed toevaporate. This evaporation step is normally performed by baking thepellicle frame in an industrial, clean oven. Whether or not theevaporation step involves baking in an oven, may not remove all of thesolvent, but it does result in the adhesive being substantially driedand non-tacky. Thus, when one step of this process is described as"permitting solvent in the adhesive to evaporate", this does notnecessarily mean that all of the solvent is evaporated.

In order to mount the pellicle membrane to the pellicle frame, solventis applied to the dried adhesive on the pellicle frame. Because thepellicle membrane is so thin, solvent control is critical. Too muchsolvent could result in a broken or wrinkled pellicle membrane as aresult of the solvent dissolving excessive amounts of the pelliclemembrane after the membrane is applied to the frame. Too little solventcould result in insufficient bonding between the membrane and theadhesive on the frame.

After the solvent is applied to the dried adhesive, the solventtypically only dissolves the outer or upper portion of the driedadhesive. This way there is sufficient adhering taking place tocompletely bond the membrane, but excessive solvent will not be presenton the adhesive to damage the working portion of the membrane whichextends across the pellicle frame. The solvent utilized in thisoperation is normally FC40, FC43 or FC77 from 3M, or a mixture of two ofthese. A single solvent may be utilized, but in practice, for bettercontrol, a 20/80% mixture of two of the solvents, such as FC40 and FC77,is used. Other ratios might be preferable for particular applications.The use of this mixture at approximately these ratios results in aformulation in which the FC77 will evaporate before the FC40, therebyresulting in greater control of the mounting process. Other solvents mayalternatively be used, but it is preferable that a mixture of at leasttwo solvents be utilized, with the solvents having differing boilingpoints, perhaps by as much as 10° C. or more.

After application of the solvent or co-solvent, the pellicle membrane isapplied. This is done after the adhesive becomes tacky, and not too muchof the solvent is left, which may vary from a few seconds to severalhours, depending on the solvents. With the noted solvents, it has beenrealized that the pellicle membrane can be applied in less than 120seconds after application of the solvent.

The pellicle membrane 14 is then mounted to the pellicle frame bypressing the membrane against the adhesive-coated frame, using aperimeter frame 15 such as that shown in FIG. 3. This perimeter frame 15is normally the frame on which the pellicle membrane 14 is fabricated.The weight of the perimeter frame 15 is normally sufficient to ensurethat the pellicle membrane 14 will be taut across the pellicle frame 12as shown in FIG. 3.

Finally, once the membrane has bonded to the adhesive, the excessportion of the membrane is cut or trimmed away, using a knife. Any roughedge which may be left is cleaned by applying a solvent, such as one ofthose described above, to the outer edge of the membrane.

In some applications, as shown in FIG. 5, it might be desirable to applya tear barrier in the form of a bead of hot melt glue 22 around theframe close to the outer edge thereof in a pattern concentric with theouter edge. The hot melt glue would normally be solidifiable fluidpolymeric material. A bead of hot melt glue could be disposed directlyover the outer edge of the flame, or could be disposed outwardly thereofas described in U.S. Pat. No. 5,254,375 and as depicted in FIG. 5, thedisclosure of such patent being incorporated herein by reference. Or, asshown in FIG. 6, the tear barrier may be in the form of a thin line oftape 24, in a similar pattern following the shape of the pellicle, whichcould be applied to the pellicle membrane. In either case, the glue 22or tape 24 would prevent a runaway tear from damaging the pelliclemembrane 14 during the trimming or cutting step, whether that operationis performed using a knife or the laser 20 described above.

A typical pellicle mounting process will now be described, using thematerials set forth above.

EXAMPLE ONE

Apply CTL-816-AP to the pellicle frame 12. Bake the frame with theadhesive thereon in an oven at 75°±1° C. for 4-5 hours or until theadhesive is dry. Prepare a mixture of FC40 and FC77 in a 20/80% ratio,and then apply this co-solvent mixture to the baked adhesive on theframe using a sponge Q-tip and wait for a few seconds while the solventdissolves the top portion of the baked adhesive so that thepreviously-applied adhesive becomes tacky. A pellicle membrane 14,fabricated of poly-perfluoro cyclo oxyaliphatic polymer as set forthabove, is then mounted to the tacky adhesive. Excess membrane is thentidied from the frame using a knife, and the peripheral edge is cleanedusing a solvent such as one of those set forth above.

Once the membrane has been mounted as described in Example One, it maybe desirable in some applications to further bond the pellicle to theframe using a heat source, and preferably a localized heat source suchas a laser, a heat gun or through the use of ultrasound technology. Asuitable laser, identified at 20 in FIG. 3, is an infrared CO₂ lasersuch as that marketed by Synrad, Inc. of Bothell, Wash. under thedesignation Series 48 laser "E" version. This laser is applied insufficient intensity and for a sufficient time that it generates heat inthe pellicle membrane and adhesive and thereby bonds the adhesive to theframe but does not dissolve the entire membrane or cause it to wrinkle.It has been determined that when a 20 watt CO₂ laser is used, it can beset at a low energy setting and have the capability to partially meltthe membrane and adhesive without breaking the membrane, thereby bondingthe membrane to the adhesive. The speed at which the laser passes aroundthe pellicle frame can provide additional control for this process. Theuse of similar materials for the membrane and the adhesive as describedabove (with the adhesive having a lower Tg than the membrane material)has been shown to provide greater control to the process.

While the laser has been identified at 20, this laser may alternativelybe a heat gun or an ultrasound application tool. Such a gun or toolwould be depicted as shown at 20 in FIGS. 3 and 4. These drawings havenot been redone showing such a gun or tool because the gun or tool wouldappear much as the laser does in those figures.

EXAMPLE TWO

Once the membrane has been mounted as set forth above, the laser isutilized to further bond the pellicle membrane 14 to the pellicle frame12, and an example of that process will now be described.

The beam in the Synrad CO₂ laser is confined to a localized area whichis normally thinner than the width of the pellicle frame. It can befocused, for example, using a lens or by using fiber optics. It ispreferable to use a beam size which is less than the width of thepellicle frame 12, typically 2 mm. A beam size of about 1.5 mm isdepicted in FIG. 1 and is used in this example. The laser is spacedabove the pellicle membrane, and the center of the laser beam isdisplaced outwardly, approximately two thuds of the way from the innerto the outer edge, as shown schematically in FIG. 3. This is to minimizethe possibility of damage to the portion of the pellicle membrane whichextends inwardly from the pellicle frame which is positioned over thephotomask. The use of this laser as described will normally result inthe surface temperature being between the Tg temperature of the adhesiveand the melting temperature of the polymer. The laser is then passedentirely around the frame, using an XY automated platen, thereby furtherbonding the membrane to the frame by causing the membrane and adhesiveto at least partially melt and bond to one another.

It is possible to expedite the pellicle assembly operation by applyingthe laser before the cutting and trimming steps set forth above. In thisinstance, following the bonding step set forth in Example Two, the focusof the laser would be further defined, and/or the intensity of the beamwould be increased, and is shifted to the outer edge of the pellicleframe as schematically depicted in FIG. 4. The laser would then beenergized to cut the excess membrane through the generation of localizedheat in the membrane, as schematically depicted at 14a in FIG. 4,instead of using the knife to perform the membrane cutting step. Thisexcess portion of membrane 14a would of course normally be supported byan outer frame, which has not been shown in the drawings. Once thislaser cutting operation is completed, solvent could again be used totrim the edge as set forth above, although in certain applications dieheat generated by the laser cutting may be such that the final solventapplication step may be eliminated.

Other variations of the preferred embodiments are intended to beencompassed by the claims which follow.

It is claimed and desired to secure by Letters Patent:
 1. A method ofassembling an optical pellicle including a pellicle frame and a pelliclemembrane comprising:selecting a pellicle frame corresponding to theshape of the desired pellicle; applying adhesive containing a firstsolvent to the pellicle frame; heating the pellicle frame to evaporatethe first solvent; fabricating a pellicle membrane; applying a secondsolvent to the adhesive on the pellicle frame, wherein the secondsolvent is a mixture of at least two solvents having boiling pointswhich differ by at least 10° C.; and mounting the pellicle membrane tothe adhesive on the pellicle frame.
 2. The method of claim 1 wherein thestep of fabricating the membrane fabricates the membrane ofpoly-perfluoro polymer containing a cyclic ether functional group. 3.The method of claim 2 wherein the step of fabricating the membranefabricates the membrane of poly-perfluoro cyclo oxyaliphatic polymer. 4.The method of claim 1 or 2 wherein the step of applying adhesive appliesa fluorinated polymer containing a cyclic ether functional group.
 5. Themethod of claim 1 or 2 wherein the step of applying adhesive applies apoly-perfluoro cyclo oxyaliphatic polymer.
 6. The method of claim 1wherein the step of fabricating the membrane fabricates the membrane ofa Teflon amorphous fluoropolymer.
 7. The method of claim 1 wherein thestep of fabricating the membrane fabricates the membrane of amorphouscopolymers of perfluoro (2,2-dimethyl-1,3 dioxole) (PDD) andtetrafluoroethylene.
 8. The method of claim 1 wherein the step ofheating the pellicle frame to evaporate the first solvent comprisesbaking the frame with the adhesive thereon at an elevated temperature.9. The method of claim 1 wherein the step of mounting the membraneinvolves mounting the membrane after at least some of the second solventhaving the lower boiling point evaporates.
 10. The method of claim 1wherein the second solvent is a fluorocarbon-based solvent.
 11. Themethod of claim 1, wherein the second solvent applied to the adhesive onthe pellicle frame is selected to at least partially dissolve theadhesive and the pellicle membrane.
 12. The method of claim 1 whereinthe step of mounting the pellicle membrane to the adhesive on thepellicle frame comprises mounting the pellicle membrane afterapplication of the solvent and after the adhesive becomes tacky.
 13. Themethod of claim 1 wherein the step of mounting the pellicle membrane tothe adhesive on the pellicle frame comprises mounting the pelliclemembrane within 120 seconds of application of the solvent to theadhesive.
 14. The method of claim 1, further comprising generating heatin the pellicle membrane and adhesive after the membrane is mounted tothe adhesive on the pellicle frame to further bond the membrane to theframe.
 15. The method of claim 14 wherein the heat is generated using alaser.
 16. The method of claim 14 wherein the heat is generated using aheat gun.
 17. The method of claim 14 wherein the heat is generated usingultrasound.
 18. The method of claim 14 wherein the heat is generated ina localized area.
 19. The method of claim 18 wherein the heat isgenerated by a beam which is thinner than the width of the pellicleframe.
 20. The method of claim 19 wherein the pellicle frame has aninner edge and an outer edge and wherein the heat is generated in themembrane at a region spaced from the inner and outer edges.
 21. Themethod of claim 20 wherein the heat is generated in the membrane at aregion closer to the outer edge than to the inner edge.
 22. The methodof claim 14, wherein the pellicle membrane has an inner edge and anouter edge, and wherein the method further comprises cutting any portionof the membrane extending over the outer edge of the pellicle framethrough the generation of localized heat in the membrane, following thestep of generating heat in the pellicle membrane and adhesive.
 23. Themethod of claim 22, further comprising narrowing the area of heatgeneration before cutting any portion of the membrane.
 24. The method ofclaim 22, further comprising increasing the intensity of the generatedheat before cutting any portion of the membrane.
 25. The method of claim23 wherein the area of heat generation is narrowed using a lens focusingsystem.
 26. The method of claim 23 wherein the area of heat generationis narrowed using fiber optics.